NICMOS was installed onboard the HST during the second
Servicing Mission (SM2) in February
1997. Prior to the SM2 launch, an extensive ground testing program was executed,
during which
the NICMOS dewar was filled with about 240 pounds of liquid nitrogen. The
nitrogen was then
solidified by passing cold helium gas through a coil located at the aft end of
the dewar. This reduced
the temperature of the nitrogen to about 40 K. During testing and storage,
passive heat inputs
caused the block of solid nitrogen to slowly warm up - an expected behavior. To
avoid reaching the
triple point at about 63 K, the block was recooled approximately every 6-8
weeks, again using cold
Helium gas circulating through the aft end cooling coil. During this process,
nitrogen gas froze onto
the cooling coil. This reduced the vapor pressure at the aft end, effectively
pumping gas from the
warmer fore end to the aft.

As the dewar was allowed to warm up, the ice at the aft end
expanded, pushing into the interior
surfaces of the dewar and deforming it. By mid-1996 the three cameras in NICMOS
were no
longer parfocal although there were good reasons to expect that they would
return to a nearly
parfocal state after a fraction of the nitrogen had evaporated on orbit. At that
time a total
deformation of about 4 mm had been observed and steps were taken to assure that
the dewar
remained flight worthy and that subsequent recooling Cycles did not stretch the
dewar further.
Also, the internal optical alignment and focus mechanism (the Pupil Alignment
Mechanism
PAM) was replaced with a version providing twice the focus range and a
demonstrated capability
for frequent movement. The PAM, originally intended to align the input beam onto
the corrective
optic and to bring NICMOS into parfocality with the WFPC2 (the only HST
instrument without
an internal focus mechanism), would be used to support a unique focus setting
for each NICMOS
camera and to switch between them routinely. After NICMOS was installed in HST,
the dewar was
planned to warm up to about 57 K. This high a temperature was never allowed to
be reached during
ground testing. The ice expansion caused by this temperature increase resulted
in an additional
dewar deformation, to the extent that one of the (cold) optical baffles made
mechanical contact
with the warmer vapor-cooled shield (VCS). The resulting heat flow caused the
ice to warm up
even more, to about 60 K, which in turn deformed the dewar more.

This unexpectedly large deformation had several undesirable
effects, the most important of which
are:

The three cameras have significantly different foci, hence parallel observations
are degraded.
The difference between the NIC1 and NIC2 foci, however, is sufficiently small
that an
intermediate focus yields good quality images in both cameras.

The NIC3 focus has moved outside of the range of the PAM. In order to enable
execution of
Cycle 7 programs that required the use of camera 3, two NIC3 campaigns were
performed in
January and June of 1998. These were periods of 2-3 weeks during which the HST
secondary
mirror was adjusted to bring the focus back into the NIC3 PAM range. During this
time, HST
performed exclusively NIC3 science, since no other HST instrument was in focus.
While it was
initially hoped that the NIC3 focus would eventually return to within the PAM
range, we no
longer expect this to happen. However, at the maximum PAM position, the
degradation in
terms of encircled energy at a 0.2" radius is only 10-15%. This is considered
sufficiently small,
and NIC3 will be offered as is in Cycle 11 and future Cycles.

The thermal short increased the heat flux into the inner shell (and therefore
the solid
nitrogen) by a factor of 2.5 and thereby reduced the lifetime of NICMOS from 4.5
to ~ 2
years. The cryogen depleted in January 1999, and NICMOS has since been
unavailable for
science operation. To enable completion of the NICMOS science program despite the
shortened lifetime, NASA and STScI adjusted the HST scheduling in such a way that
NICMOS observations were assigned 40-50% of the total observing time in Cycle 7.
Moreover, a second Call for Proposals (CP) for additional NICMOS science was
issued in
summer 1997. The proposals were put through the peer review process, and the
full science
program was executed before the cryogen depletion.

Following the recomemndations of the Independent
Science Review committee, NASA has
developed the concept of the NICMOS Cooling System (NCS) in
order to restore and conserve an
infrared capability on HST. The NCS is a mechanical cryocooler that will
reenable NICMOS
operation by cooling the instrument to temperatures around 75-86 K. This range
is cool enough to
allow operation of the NICMOS detectors, but is significantly higher than during
Cycle 7 science
operations. Therefore, many NICMOS parameters will be different from Cycle 7. In
order to
estimate the impact of the higher operating temperature on NICMOS sensitivity, a
large
monitoring program was executed during the instrument warmup following the cryogen
exhaustion. The execution, data products, and results of the warmup analysis are
described in detail
in NICMOS ISR 99-001. A
thorough discussion of a dark current anomaly observed during the
warmup can also be found in this paper.

In March 2002, the NCS was successfully installed on the NICMOS, and the
instrument is now performing
even better than it did during Cycle 7. Now operating at a slightly warmer
temperature, 77.1 K, the Instrument is
more sensitive than it was during Cycle 7 and the temperature is also more
constant. Furthermore, the focii for all three
cameras have remained stable, and the NIC3 focus has moved in the positive
direction relative to its Cycle 7 position, so as to be nearly in focus!
Details about the operation of the NICMOS under NCS, as well as recommended
strategies for observing proposals can be found in the
Cycle
17 Instrument Handbook.